1. Field of the Invention
This invention relates to an apparatus for suppressing vibrations and quaky movements in the travel of mobile or automotive type cranes such as rough terrain cranes.
2. Prior Art
As illustrated in FIG. 3, mobile type cranes generally have a boom 3 pivotally supported on a vehicle body 2 which is supported on driven wheels 1, for pivoting movements about a horizontal shaft 5 through a boom uplifting cylinder 4. With a mobile crane of this type, it is usually experienced that the vehicle body 2 is put in vibrations or quaky movements during travel due to undulations or irregularities on road surfaces or due to abrupt accelerations or decelerations of the vehicle body 2, putting the boom 3 and other attachments also in quaky movements to magnify the vibrations and quaky movements of the vehicle body 2 itself, causing of ride discomfort to the operator on the vehicle.
For the purpose of damping such vibrations in travel, there has been known in the art an apparatus as disclosed in Japanese Laid-Open Patent Application No. 59-182195. As shown in FIG. 4, this prior art apparatus employs a damper mechanism 19 which is built in a boom uplifting cylinder 18, a counter-balancing valve 12 provided in a conduit 13 in communication with an oil chamber 181 which holds the load of the cylinder 18, and an electromagnetic change-over valve 16 and a shuttle valve 15 which are provided between the conduit 13 and a conduit 17 which is connected to the other oil chamber 182 or a conduit 14 which is in communication with a direction control valve 11.
According to the prior art apparatus, if the direction control valve 11 is switched to a boom-up or boom-down position when the change-over valve 16 is in position A, the oil pressure is supplied to the oil chamber 181 or 182 of the cylinder 18 to expand or contract the cylinder 18 for lifting up or down the boom. If the change-over valve 16 is switched to position B, the conduit 13 is communicated with the conduit 17 through the electromagnetic valve 16 and shuttle valve 15, forming a closed circuit through the oil chambers 181 and 182 and the oil chamber 191 of the damper mechanism 19 to thereby suppress quaky movements relative to the vehicle body 2 when the vehicle is in travel.
With this prior art arrangement, even if the change-over valve 16 is returned to position A after a vehicle travel operation with the change-over valve 16 in position B to perform the vibration suppressing function, the ball 151 of the shuttle valve 15 is retained in the right position, which is shown in the drawing, by the load pressure produced in the vehicle travel operation, holding the conduit 14 in closed state and sealing the load pressure in the oil chamber 182.
Therefore, in order to start a crane operation after a vehicle travel, it is required not only to return the change-over valve to position A but also to switch the direction control valve 11 once to a boom-down position to supply the oil pressure from the pump 10 to the conduit 14 to thereby move the ball of the shuttle valve 15 to the left while supplying the oil pressure to the oil chamber 182 through the conduit 17 to contract the cylinder 18 to its stroke end. Thereafter, the direction control valve 11 has to be returned to the neutral position. Needless to say, these resetting operations are very troublesome.
If the direction control valve 11 is directly switched to the boom-up position to start a crane operation, neglecting the above-mentioned boom-up operation, the oil pressure from the pump 10 will be supplied to the oil chamber 181 through the conduit 13 to stretch the cylinder 18. However, at this time, the oil which tends to flow into the conduit 17 from the oil chamber 182 is blocked by the shuttle valve 15, finding no way to flow into the conduit 14 or toward the tank. Namely, the cylinder 18 is stretched in the fashion of a ram cylinder, and a pressure equivalent to the load holding pressure in the oil chamber 181 also prevails in the oil chamber 182. It follows that the effective pressure receiving area of the piston 183 in the oil chamber 181 becomes a small area corresponding to the sectional area of the rod 184, and therefore the load holding pressure in the oil chamber 181 is increased to an abnormally high level. If the load holding pressure exceeds the preset relieving level of the counter-balancing valve 12, an overload relief valve is opened, contracting the cylinder 18 contrary to the operator's intention and causing an abrupt drop of the boom 3 which imposes a great shock on the vehicle body 2.
On the other hand, in order to perform the vibration suppressing function effectively on a mobile type crane, it is necessary to expand the cylinder 4 (denoted at 18 in FIG. 1) slightly from its fully contracted state as shown in FIG. 4, holding the fore end of the boom at an appropriate level from the ground surface, that is to say, at a level higher than a lower limit height H.sub.0 and lower than a limit height H.sub.2 as stipulated in traffic regulations. That is to say, the appropriate boom height H1 for the vibration suppressing function should be H.sub.0 &lt;H.sub.1 &lt;H.sub.2.
In the above-described prior art apparatus, however, if the cylinder 18 is expanded slightly from the fully contracted state after a crane operation to hold the boom at the appropriate height H.sub.1 and then the change-over valve 16 is switched to position B to form a closed circuit, the load holding pressure in the oil chamber 181 is led to and accumulated in the oil chamber 191 of the damper mechanism 19, contracting the cylinder 18 in a degree corresponding to the oil compression volume in the oil chamber 191. This causes an abrupt downfall of the boom 3 which imposes a great shock on the vehicle body 2. In addition, the boom 3 is dropped below the appropriate height H.sub.1, making it difficult to perform the vibration suppressing function effectively.